The present invention relates to a failure detection apparatus for a cooling system of an internal combustion engine, for detecting failure of a thermostat provided in the cooling system for the internal combustion engine.
In general, a cooling system for a water-cooled internal combustion engine for a vehicle includes a radiator for lowering the temperature of cooling water, a cooling water pipe for circulating the cooling water between the engine and the radiator, and a thermostat (temperature-sensitive switch valve) for opening and closing the cooling water pipe. The thermostat operates as follows: when the cooling water temperature is low, e.g. at starting of the engine, the thermostat closes the cooling water pipe to thereby accelerate the rise in temperature of the engine, and when the cooling water temperature becomes high, the thermostat opens the cooling water pipe to circulate the cooling water from the engine to the radiator to thereby allow the cooling water to be cooled in the radiator.
A method of detecting failure of the thermostat is disclosed in Japanese Patent Laid-open No. 2000-8853, for example. According to the method disclosed in this publication, an estimated value CTW of a cooling water temperature (which value will be hereinafter referred to as xe2x80x9cestimated water temperaturexe2x80x9d) is calculated according to a temperature condition at starting of the engine and engine operating conditions after starting of the engine. In the case that an actual cooling water temperature TW does not reach a normality determination value at the time the estimated water temperature CTW has reached a failure determination value, it is determined that the thermostat has failed (the thermostat has not properly closed). On the other hand, in the case that an averaged vehicle speed is higher than or equal to a predetermined value VPSAVTRM at the time the actual cooling water temperature TW reaches the normality determination value before the estimated water temperature CTW has reached the failure determination value, it is determined that the thermostat is normal. Further, if the averaged vehicle speed is lower than the predetermined value VPSAVTRM at the time the actual cooling water temperature TW reaches the normality determination value, a radiator fan is forcibly operated for a predetermined time period, and the determination of whether or not the thermostat has failed is then performed according to the actual cooling water temperature TW after the operation of the radiator fan.
However, the above conventional method has a problem such that if the average of vehicle speeds is lower than the predetermined value VPSAVTRM at the time the actual cooling water temperature TW has reached the normality determination value, the determination that the thermostat is normal cannot be immediately made and the time required for this determination becomes long. Further, the radiator fan must be operated in this case, so that there is a possibility of degradation in fuel economy or degradation in heater performance due to operation of the radiator fan.
It is accordingly an object of the present invention to provide a failure detection apparatus which can perform rapid and accurate determination even when the vehicle speed is low and can prevent degradation in fuel economy and degradation in heater performance due to operation of the radiator fan.
In accordance with the present invention, there is provided a failure detection apparatus for a cooling system of an internal combustion engine. The cooling system has a radiator (21), a cooling water pipe (22) for circulating a cooling water in the engine to the radiator, and a thermostat (23) provided in the cooling water pipe for opening and closing the cooling water pipe. The failure detection apparatus detects failure of the thermostat and comprises vehicle speed detecting means (18), operating condition detecting means, estimated cooling water temperature calculating means, and determining means. The vehicle speed detecting means detects a vehicle speed (VPS) of a vehicle driven by the engine. The operating condition detecting means detects an operating condition of the engine, and includes cooling water temperature detecting means (10) for detecting a temperature (TW) of the cooling water. The estimated cooling water temperature calculating means calculates an estimated cooling water temperature (CW) of the engine according to the operating condition detected by the operating condition detecting means. The determining means compares the estimated cooling water temperature (CW) and a determination threshold (CTWOKJD) set according to the vehicle speed at the time the cooling water temperature (TW) detected by the cooling water temperature detecting means has exceeded a predetermined water temperature (TWJUD), to thereby determine that the thermostat is normal.
With this configuration, the estimated cooling water temperature (CTW) and the determination threshold (CTWOKJD) set according to the vehicle speed are compared at the time the detected cooling water temperature (TW) has exceeded the predetermined water temperature (TWJUD), to thereby determine that the thermostat is normal. Accordingly, by using the determination threshold according to the vehicle speed, it is possible to rapidly and accurately determine that the thermostat is normal even when the vehicle speed is low. As a result, it is possible to prevent a degradation in fuel economy or a degradation in heater performance due to the operation of a radiator fan.
Preferably, the determining means determines that the thermostat is normal when the estimated cooling water temperature is lower than or equal to the determination threshold.
Preferably, the determination threshold (CTWOKJD) is set to a value decreasing with a decrease in the vehicle speed.
Preferably, the cooling system failure detecting device further comprises average vehicle speed calculating means for calculating an average vehicle speed (VPSAVE) after completion of starting of the engine, wherein the determination threshold (CTWOKJU) is set according to the average vehicle speed (VPSAVE).
Preferably, the cooling system failure detecting device further comprises average vehicle speed calculating means for calculating an average vehicle speed after completion of starting of the engine, wherein the determining means determines that the thermostat is normal when the estimated cooling water temperature (CTW) is lower than a predetermined estimated water temperature (CTWJUD) higher than the predetermined water temperature (TWJUD) and the average vehicle speed (VPSAVE) is higher than or equal to a determination vehicle speed (VPJUD) at the time the detected cooling water temperature (TW) has exceeded the predetermined water temperature (TWJUD).
Preferably, the determination vehicle speed (VPJUD) is set according to an initial cooling water temperature (TWINI) at starting of the engine.
Preferably, the estimated cooling water temperature calculating means comprises thermal load parameter calculating means for calculating a thermal load parameter (TITTL) indicating a degree of raising the cooling water temperature, wherein the estimated cooling water temperature calculating means calculates the estimated cooling water temperature (CTW) according to the thermal load parameter (TITTL) and an initial cooling water temperature (TWINI, KDCTW) at starting of the engine.
Preferably, the thermal load parameter calculating means comprises generated heat quantity calculating means for calculating a generated heat quantity integrated value (TIMTTL) indicating an integrated value of a heat quantity generated from the engine after starting of the engine and cooling loss calculating means for calculating a cooling loss integrated value (CLTTL) indicating an integrated value of a heat quantity lost by a heater in the vehicle and a heat quantity lost by a wind after starting of the engine, wherein the thermal load parameter calculating means calculates the thermal load parameter (TITTL) according to the generated heat quantity integrated value (TIMTTL) and the cooling loss integrated value (CLTTL).
Preferably, the generated heat quantity calculating means calculates the generated heat quantity integrated value (TIMTTL) according to an intake air amount per unit time of the engine, and the cooling loss calculating means calculates the cooling loss integrated value (CLTTL) according to an estimated water temperature rise (DCTW) after starting of the engine and the vehicle speed.
Preferably, the cooling system failure detecting device further comprises determination permitting means for permitting determination by the determining means when the engine is started in the condition where the engine is cooled to a temperature substantially equal to an outside air temperature.